Process for nitriding unalloyed or low-alloy steel

ABSTRACT

A process for the nitriding of unalloyed or low-alloy steel in an ammonia-containing atmosphere which comprises initially maintaining the steel in this atmosphere at a temperature above the Alpha - gamma transformation point (about 585* C) for a period of the order of hours and thereafter treating the steel body in a nitrogen-containing atmosphere for a period of hours at a temperature below Alpha - gamma transformation point of the iron-nitrogen system. Preferably, the first stage is carried out at a temperature of about 650*C with the residual ammonia content of the throughflowing furnace gases of at least 10% while the second stage is carried out with a residual ammonia content of 45 to 60% at a temperature of about 570*C for a period of two to four hours.

United States Patent [1 1 Brugger et al.

1 1 PROCESS FOR NITRIDING UNALLOYED OR LOW-ALLOY STEEL [75] Inventors: Hans Brugger; Helmut Mallener,

both of Friedrichshafen, Germany [73] Assignee: Zahnradfabrik Friedrechshafen AG., Friedrechshafen, Germany [22] Filed: Apr. 26, 1973 [21] Appl. No.: 354,731

[30] Foreign Application Priority Data Apr. 29, 1972 Germany 2221169 [52] U.S. Cl. 148/165, 148/166 [51] Int. Cl. ..C23c11/l6,C23c 11/18 [58] Field of Search 148/16, 16.5, 16.6

[56] References Cited UNITED STATES PATENTS 1,965,798 7/1934 Egan 148/166 2.472.320 6/1949 Vennerholm et al. 148/165 FOREIGN PATENTS OR APPLICATIONS 696,688 9/1953 Great Britain 148/165 711,848 7/1954 Great Britain 148/165 1 Mar. 11, 1975 OTHER PUBLICATIONS Metal Progress, Nov. 1949, pgs. 651-656.

Primary E.\'aminerC. Lovell Attorney, Agent. or Firm-Karl F. Ross; Herbert Dubno ABSTRACT A process for the nitriding of unalloyed or low-alloy steel in an ammonia-containing atmosphere which comprises initially maintaining the steel in this atmosphere at a temperature above the (Jrtransformation point (about 585C) for a period of the order of hours and thereafter treating the steel body in a nitrogencontaining atmosphere for a period of hours at a temperature below a-y transformation point of the ironnitrogen system. Preferably, the first stage is carried out at a temperature of about 650C with the residual ammonia content of the throughflowing furnace gases of at least 10% while the second stage is carried out with a residual ammonia content of 45 to 60% at a temperature of about 570C for a period of two to four hours.

6 Claims, No Drawings PROCESS FOR NITRIDING UNALLOYED OR LOW-ALLOY STEEL FIELD OF THE INVENTION The present invention relates to a method of or a process for the nitriding of low-alloy or unalloyed steel, e.g. so-called soft steel, to increase the wear resistance, abrasion resistance, surface hardness and' strength of at least the surface and edge zones of the steel article. More particularly, the invention relates to improvements in the nitriding of such steel whereby disadvantages encountered heretofore are obviated.

BACKGROUND OF THE INVENTION The nitriding of steel has long been recognized as a means for increasing the hardness and improving other properties of surface zones of a steel body. The nitriding process involves a diffusion of nitrogen, generally from an environment containing nitrogen in an available form, e.g. an ammonia-containing gas stream. This process yields a hard wear-resistant case and is advantageous where the surface is to be exposed to wear, abrasion or the like.

In general, nitriding has been carried out heretofore at a temperature below the 01-7 transformation of the iron-nitrogen system, i.e. under a temperature of 585C. The most important advantage of a treatment below the transformation temperature, of course, is a general freedom from distortion in the ultimate product.

Because of the relatively low nitriding temperature, the nitrogen diffuses into the steel only relatively slowly and, since the solubility of the a for nitrogen is slight at best, it is necessary to provide surface-hardening times with conventional nitriding techniques of extremely long duration, e.g. 50 hours or above.

Investigations into. reducing the nitriding time have concentrated upon high-alloy steel and elevated nitriding temperatures and even with these variations satisfactory results have not been attained. In the first case, the nitrided steels give a maximum hardness at temperatures in the vicinity of 500C because of the formation ofspecial nitrides, and in the second case distortion of the steel article is promoted. In choosing the nitriding temperature it is important to maintain strength retentivity so that the basic tensile and other strength characteristics of the body will not be lost. Nitriding at temperatures above the 01- transformation (in the ironnitrogen system) has not to date been able to provide a successful solution to the aforementioned problems and hence the principal effort has been directed to the lower temperature long-duration process.

Attempts have also been made to increase the nitrogen penetration of the diffusion zone of a steel body by activating the zone of the diffusing substances by use of high-energy fields. For example, high frequency signals, corona discharge and glow discharge techniques have been used at the diffusion surface although without complete success.

It may be mentioned that so-called soft" steels (i.e.

trace amounts of manganese, silicon, aluminum. chromium and molybdenum, have not been successfully nitrided economically with retention of strength.

OBJECTS OF THE INVENTION It is the principal object of the present invention to provide an improved method of or process for the nitriding of low-alloy or non-alloyed steel whereby the aforementioned disadvantages can be obviated.

Another object of this invention is to provide a nitriding system for reducing the duration of the nitriding treatment and providing an improved product.

DESCRIPTION OF THE INVENTION preferably at a temperature of about 650C. The sec- 0nd stage, carried out after cooling of the article, also makes use of an ammonia-containing atmosphere and is effected at a temperature below the a-ytransition non-alloyed or low-alloy steels), as distinct from the higher-alloy nitriding steels, are generally nitrided only for limited periods because they suffer with prolonged exposure to the nitriding environment and temperature a substantial loss in strength. Thus far, therefore, lowalloy steels and especially those containing no or only temperature, preferably at a temperature of 570C.

During the higher temperature of the first stage, diffusion takes place at a greater rate and penetration of nitrogen into the case of the steel article increases in depth. It has been found to be advantageous to provide a slow cooling between the first and second stages, e.g. by permitting the steel bodies to cool the second-stage temperature without forced heat exchange. Advantageously, the steel bodies are permitted to cool within the furnace which sustains no circulation of gases dur ing the cooling process. In general, cooling may be carried out over a period of hours. The slow cooling precludes distortion of the nitrogen-containing case of the steel body.

It has been found advantageous to provide a carboncontaining nitriding medium in the second stage of the process and to therefore introduce into the gas stream a compound containing both carbon and nitrogen, preferably methylamine, in an amount of 5 to 20 volume of the ammonia which would normally be supplied. The compound releases carbon to the steel workpieces and promotes the formation of the e-carbonitride which has greater strength and wear resistance than the ynitride formed in the steel articles when the atmosphere during the two stages is carbon-deficient.

As previously mentioned, the first stage results in a rapid and considerable diffusion of nitrogen into the steel body whereas the slow cooling through and below the my transformation temperature results in a saturation of the diffusion zone with nitrogen and carbon, at least at the junction of this zone with the remainder of the body.

The first temperature stage is carried out in the realm of the maximum solubility of nitrogen in the y mixed crystal phase (about 65 0C). At a certain nitrogen concentration, the layer below the junction zone is transformed into nitrogen austenite ('y=mixed crystals). The thickness of this layer, hereinafterreferred to as the separating layer), is a function of the nitrogen level of the environment and hence the supply of nitrogen to the metal surface, and a function of the treatment time at this temperature. The thickness can range from several microns to several tenths of a millimeter. The separating or intermediate layer has a higher strength than the core lattice of the steel body and ensures an effective transition of the surface layer or case to the core lattice even under high specific loading, as tests have demonstrated using rolling processes.

The nontransformed region beneath the intermediate pr separating layer comprises a mixed crystals and has, at a temperature of 650C, a diffusion coefficient which is abouta factor of 5 greater than the diffusion coefficient at temperatures below the a-y transformation point (about 585C) so that greater penetration depths for nitrogen per unit time may be noted.

The slow cooling to the second stage 585C), in which the temperature during the cooling interval lies for the greater part thereof at or below the a-y transformation temperature (585C) reduces the tendency toward transformation stresses. Furthermore, it ensures a desirable grain structure in the surface zones of the steel body inasmuch as time is provided for diffusion equalization during cooling and during the retention of the body at the second stage temperature. Any distortion is thus within the limits of conventional nitriding processes.

The second nitriding stage is carried out, as indicated previously, at temperatures below the 01-3 transforma.

a nitrided layer in the absence of diffused carbon.

Each of nitriding temperature stages is associated with a characteristic dissociation degree of the ammonia and in the first temperature stage (operating at about 650C) the degree of dissociation should be at least 0.8. This is achieved by feeding ammoniacontaining gas into the system through the furnace with the ammonia level adjusted so that the atmosphere upon treatment of the steel body has a residual ammonia content in excess of The upper limit of the residual ammonia content, a function of the size of the furnace, is determined by that which is necessary to obtain uniform nitriding of the body at the speed at which the ammonia gases flow through the system. ln the second temperature stage (operating at about 570C) the dissociation degree should range between 0.25 and 0.35, corresponding to a residual ammonia content of 45 to 60%.

Nitriding is carried out in the first stage for a period of at least 2 hours and preferably between 2 and 8 hours or more while the second-stage nitriding is carried out for'2 to 4 hours. The cooling of the body between the first and second stages is carried out at a low rate, as noted earlier, and preferably over a period of hours although this time it is not critical. It has been found to be practical to allow the body to cool naturally in contact with the furnace gases which are not circulated or cooled, especially during this period. The heat loss from the body is thus transferred out of the system through the furnace walls. The cooling of the body from the second-stage treatment may be as rapid as is desired and may be effected by gas or liquid quenching. Preferably, cooling is effected from the second-stage temperature to ambient by circulating gas at ambient temperature through the furnace. Advantageously, this gas is nitrogen.

It has also been observed that, with the addition of a carbon carrier to the environment of the second stage,

carbon is picked up by the body in the interfacial zone with formation of e-carbonitride, as noted. A portion of the 'y'nitride produced by the previous treatment at a temperature above the a-y transformation point, he comes unstable in the presence of carbon and, by carbon substitution, is converted into the e-carbonitride. The quantity of the carbon carrier methylamine in the second stage may be between 5 and 20 volume percent of the requisite ammonia level.

The process according to the invention is preferably used for the nitriding of structural elements of unalloyed or low-alloy structural steel which may be used in the same manner as case-hardened and tempered steels and have a higher wear resistance and strength retention. The starting material may be tempered or normalized, e.g. in a soaking pit or the like. Because of the deep nitrogen diffusion and the higher surface strength of the steel body it is possible to avoid tempering the steel prior to nitriding. It is moreover desirable to use a steel in which the core structure and strength is improved by heating to temperatures of 650C for periods of the order of those stated for the first stage.

When refrence is made herein to non-alloyed or lowalloyed steels, I intend to thus describe steels having a carbon content of, say, 0.20 0.45%, a manganese content less than 0.35%, a silicon content less than 0.15%. an aluminum content less than 0.75%, a chromium content of less than 0.80%and a molybdenum content of less than 0.10%, the percentages being all given by weight and the balance being iron. Preferably, the total SPECIFIC EXAMPLE A steel body having the following consitiuents in per cent by weight carbon 0.25, manganese 0.8, silicon 0.1, aluminum less than 0.05, chromium up to 1, molybdenum up to 0.5, nickel up to 0.5, the balance iron, in the form of a bar is heated to 650 C. and maintained thereat for 4 hours in a mantle furnace with a stream of ammonia flowing at a rate such that the residual ammonia content is 25 per cent by volume.

The steel bar is permitted to cool in the furnace to 570 C. for 2 hours and is maintained thereat for a further 2 hours and is treated with a nitriding atmosphere containing 50 per cent residual ammonia with about 15 per cent of the ammonia content methylamine. Treatment is terminated by normally cooling the bar to room temperature and flushing the furnace with nitrogen. An effective nitride case of 0.6 to 0.8 mm anda white layer of 0.04 mm thickness was provided on the bar.

1 claim:

l. A process for nitriding a body of low-alloy or nonalloyed steel which comprises treating said body in a first stage for a period of about 2 hours to 8 hours at a temperature of about 650 C, said temperature being above the 01-7 transformation temperature in the ironnitrogen system, in an ammonia-containing gas and sufficient to effect nitrogen diffusion into at least a surface 6 zone of said body; thereafter slowly cooling said body; bon carrier is a compound of nitrogen and carbon. and subsequently treating said body with an ammonia- 4. The process defined in claim 3 wherein said comcontaining gas Stream at a temperature of about 570C pound is methylamine and is provided in an amount of for a Period of 2 to 4 hours Said temperature being 5 to volume percent of the requisit ammonia in said below the oz-y transformation point, to effect equalization in said zone, the gas stream in said first stage having a residual ammonia content of at least 10% by'volmm and the gas stream in Said Second Stage having a is cooled in said second stage by passlng a coolmg gas residual ammonia content of 45 to 60% by volume. thereover- 2, Th process d fi d i l i 1 h i id gas 10 6. The process defined in claim 5 wherein said coolstream in said second stage contains a carbon carrier. g gas S trogen- 3. The process defined in claim 2 wherein said car- 5 second stage.

5. The process defined in claim 4 wherein said body 

1. A PROCESS FOR NITRIDING A BODY OF LOW-ALLOY OR NON-ALLOYED STEEL WHICH COMPRISES TREATING SAID BODY IN A FIRST STAGE FOR A PERIOD OF ABOUT 2 HOURS TO 8 HOURS AT A TEMPERATURE OF ABOUT 650*C, SAID TEMPERATURE BEING ABOVE THE A-Y TRANSFORMATION TEMPERATURE IN THE IRON-NITROGEN SYSTEM, IN AN AMMONIACONTAINING GAS AND SUFFICIENT TO EFFECT NITROGEN DIFFUSION INTO AT LEAST A SURFACE ZONE OF SAID BODY, THEREAFTER SLOWLY COOLING SAID BODY, AND SUBSEQUENTLY TREATING SAID BODY, WITH AN AMMONIA-CONTAINING GAS STREAM AT A TEMPERATURE OF ABOUT 570*C FOR A PERIOD OF 2 TO 4 HOURS, SAID TEMPERATURE BEING BELOW THE A-Y TRANSFORMATION POINT, TO EFFECT EQUALIZATION IN SAID ZONE, THE GAS STREAM IN SAID FIRST STAGE HAVING A RESIDUAL AMMONIA CONTENT OF AT LEAST 10% BY VOLUME AND THE GAS STREAM IN SAID SECOND STAGE HAVING A RESIDUAL AMMONIA CONTENT OF 45 TO 60% BY VOLUME.
 1. A process for nitriding a body of low-alloy or non-alloyed steel which comprises treating said body in a first stage for a period of about 2 hours to 8 hours at a temperature of about 650* C, said temperature being above the Alpha - gamma transformation temperature in the iron-nitrogen system, in an ammonia-containing gas and sufficient to effect nitrogen diffusion into at least a surface zone of said body; thereafter slowly cooling said body; and subsequently treating said body with an ammonia-containing gas stream at a temperature of about 570*C for a period of 2 to 4 hours, said temperature being below the Alpha - gamma transformation point, to effect equalization in said zone, the gas stream in said first stage having a residual ammonia content of at least 10% by volume and the gas stream in said second stage having a residual ammonia content of 45 to 60% by volume.
 2. The process defined in claim 1 wherein said gas stream in said second stage contains a carbon carrier.
 3. The process defined in claim 2 wherein said carbon carrier is a compound of nitrogen and carbon.
 4. The process defined in claim 3 wherein said compound is methylamine and is provided in an amount of 5 to 20 volume percent of the requisit ammonia in said second stage.
 5. The process defined in claim 4 wherein said body is cooled in said second stage by passing a cooling gas thereover. 